BERLIN (Reuters) - A Tunisian man and his German wife who bought ricin and tested the lethal toxin on a hamster have been charged with plotting Islamist-motivated attacks using a biological weapon, German prosecutors said on Thursday.

The GBA Federal Prosecutor's Office said that 30-year-old Sief Allah H. and 43-year-old Yasmin H. had acquired knowledge on how to turn ricin into a weapon and ordered 3,300 grains of the poison, which can be found in castor beans, online.

Sief Allah H. was arrested in Cologne in June and police detained Yasmin H. a month later. The couple also face charges of planning attacks using metal balls and homemade explosives as well as seeking membership of Islamic State.

Prosecutors did not say if the two suspects had a chosen a date or location for their planned attacks.

Germany has been on high alert since December 2016, when an Islamist militant from Tunisia killed 12 people by driving a truck into crowds at a Christmas market in Berlin.

Prosecutors last month charged a Russian man who knew the perpetrator of the Christmas market attack with plotting to bomb another target in Germany.

On Thursday, they said Sief Allah H. and Yasmin H. started plotting for attacks in 2018 after Sief Allah H. twice failed to cross into Syria from Turkey a year earlier.

Once he returned to Germany, he started posting Islamic State propaganda on the internet, the GBA said.